Situation update Friday 4 April
An update on rainfall and river flow from our Environmental Science team.
West of Tākaka, rainfall has ranged between 226 and 423 mm since 2/04/2025. Some Golden Bay rainfall sites have had the following amounts:
- Anatoki at Happy Sams: 423 mm
- Anatoki at Paradise: 397 mm
- Waingaro at Hanging Rock: 307 mm
- Aorere at Perry Saddle: 309 mm
On top of what rainfall has already fallen, we are expecting another 50 to 90 mm up until 11am this morning. Higher intensities may be up to 20 mm/hr. The larger Golden Bay rivers peaked late last night at flows below an annual flood. East of Tākaka at Canaan received 419 mm and the highest rainfall intensities of around 30mm/hr for several hours.
The Motueka and Riwaka Valleys have had substantially less rain (less than 100 mm), therefore river flows are predicted to be well below annual flood flows and peaking around mid-day today.
In the Richmond ranges south of Nelson, there has been around 153 mm up the Lee and Roding catchments. Another 60 to 100 mm is forecasted this morning with possible intensities up to 20 mm/hr. Wairoa flows peaked early this morning, but with the forecasted rain still to come, there will be a similar second peak later this morning. Peak flow is less than an annual flood.
In the hills behind Nelson, there has been 230 mm up the Brook at Third House and 223 mm at the Maitai Dam. Expect another 60 mm this morning on top of what we have already had, with intensities at around 10-13mm/hr.
While the Maitai peaked early this morning at an annual flood flow, there will be a second peak associated with the additional rain around mid-morning again at approximately the same flow as before.
So the weather has pretty much played out as forecasted so far, with flows at annual flood or less. While we do have some more rain coming with moderate intensities (as forecasted), the flows are still going to be around or less than annual flood.
Weather warning
Our Environmental Science team have an update following the MetService update this morning, with the overall message remaining fairly similar to yesterday. MetService Heavy Rain Warnings remain at the Orange threshold.
MetService forecast for Tasman northwest of Motueka has been extended by 1 hour to 11am Friday and to expect a further 300 to 400mm about the ranges and 110 to 160mm elsewhere during this time. As of mid-morning, rain gauges in the ranges have recorded 100 to 130mm, and this additional amount would bring the overall total to 500 to 550mm.
The peak rainfall rates have reduced slightly to be 20 to 30mm/hr. Based on current rainfall forecasts, expect moderate flooding (annual return period) in the larger rivers (Aorere, Waingaro, Anatoki, and Tākaka) with rivers likely to peak late Thursday evening.
There is a low possibility of flow across Waitapu Splash late Thursday evening. There remains a possibility of slipping impacting the roads on hills east of Tākaka, including the roads into the Abel Tasman National Park and Tākaka Hill.
For the large rivers of Motueka River and Riuwaka River, expect moderate flooding (annual) based on the current forecast.
A Heavy Rain Watch remains in place for the plains between Motueka and Nelson.
MetService forecast for Bryant and Richmond Ranges including high hill country behind Nelson and Richmond has been extended by 1 hour to noon Friday and to expect a further 200 to 250mm in the ranges and 100 to 150mm elsewhere. As of mid-morning, 20 to 25mm rainfall has been recorded in the ranges.
MetService have reduced the peak rainfall rates slightly to 15 to 25mm/hr, with the possibility of heavier rain from 6pm Thursday. Based on current rainfall forecasts expect moderate flooding (annual to 5-year return period) in the larger rivers, including Maitai River and Waimea River, with rivers likely to peak late Friday morning.
However, the possibility remains for higher rainfall intensities and event duration to extend and this scenario could quickly change and it would be prudent to plan with this in mind. As the rainfall intensities have reduced slightly, slipping is less likely to occur but is still possible in prone areas with saturated ground and higher rainfall intensities occurring toward the end of the event.
This northerly weather event is bringing warm moist air from the tropics. As always, forecasts involving rain and floods require an extra margin for safety and it would be prudent to take a conservative approach and plan for higher floods than suggested, especially if flooding consequences are severe for you.
We are not anticipating storm surge as the low-pressure system remains south and west of the region. The wind direction is north-easterly however, and larger waves are likely within the normal beach zone. There is a 3.7m high tide for Nelson at 3pm on Friday.
There remains a possibility of slipping impacting the Whangamoa and Rai Valley Roads and if you have time-critical travel you might consider driving the long way toward Blenheim via Wairau Valley Road.
Keep up to date with the latest MetService warnings here(external link).